U AP TER 1 THE NATURE OF WEATHER Nhe weather concerns everyone and has some effect on nearly every human activity It occurs within the atmosphere the mixture of gases that com pletely envelops Earth Weather is defined as the momentary day to day state of the atmosphere over any place on Earth's surface Climate on the other hand refers to weather averaged over a long period The basic atmo spheric conditions that make up the weather include precipitation humidity temperature pressure cloudiness and wind The air is constantly in movement There also is a continuous exchange of heat and mois ture between the atmosphere and Earth's land and sea surfaces These ever changing condi tions can be scientifically analyzed The science of observing and predicting the weather is known as meteorology THE ATMOSPHERE AND ITs MoVEMENTS Air is compressed by its own weight so that about half the bulk of the atmosphere is 10
THE NATURE OF WEATHER squeezed into the bottom 35 miles 5 6 kilo meters The bottom layer of the atmosphere the troposphere is the site of almost all the world's weather Above its turbulence and storminess is the calmer stratosphere which has little moisture and few clouds Underlying the great variety of atmo spheric motions is a pattern of large scale air movement over Earth The basic cause of these planetary winds or general circulation of the atmosphere is that the sun heats the air over the Equator more than it does the air over the poles The heated air over the equatorial regions rises and flows generally polewardſ in both the Northern and Southern hemispheres In the polar regions the air cools and sinks and from time to time it flows back toward the Equator HIGH AND Low PRESSURE BELTS The upward movement of air results in a belt of low pressure in the tropical regions astride the Equator On either side at about 30° N latitude and 30° S latitude is a belt of high pressure formed as the upper level flow of air from the Equator sinks to the surface From each of these subtropical high pressure II
INVESTIGATING EARTH's WEATHER The effect of the Coriolis force Nonrotating Earth launch site North Pole people Europe Africa Equator 60 45 30° 15 15 30° 45°60 target target Earth rotating 15° each hour Otoed North Pole launch site Europe l Africa Equator 75 60 45 30 150 1530 45 actual destination target actual destination 15 target Path of rotation The Coriolis force deflects the path of an object moving from a pole to the Equator It does the same to winds Encyclopaedia Britannica Inc I2
THE NATURE OF WEATHER belts surface winds blow outward toward both the Equator and the poles The Coriolis force a result of Earth's rotation deflects the winds to the right of the winds direction of motion in the Northern Hemisphere and to the left of their direction in the Southern Hemisphere This produces a belt of tropi cal easterly winds winds blowing from east to west and two belts of midlatitude west erly winds blowing from west to east one in each hemisphere Like the tropical easterlies or trade winds the surface winds from the poles are also deflected to the west Where these polar easterlies meet the westerly winds in each hemisphere at about 60° latitude a belt of low pressure girdles Earth WIND AND PRESSURE PATTERNS This arrangement of Earth s wind and pres sure belts varies somewhat with the time of the year They shift northward during the Northern Hemisphere's summer and south ward during the Southern Hemisphere's summer Both the continuity of the pres sure belts and the prevailing directions of the winds are also modified greatly by the 13
